Go to the database page of a plant that you have/want and scroll down to find a cobalt blue box that says "add this plant to your list." There is one in the left column and one at the bottom of the database entry. After you have one plant entered, a link to your plant list should show up on your profile page.

There is also a "Plant Lists" tab in the left blue column of all ATP pages. You can click that and the page will have text saying "View and manage your list" at the very top. It is very easy to overlook, though.